I. Field of the Invention
This invention relates to an improved method for the sizing of paper products. More particularly, the invention relates to an improved method for sizing paper and paperboard products under specified conditions using a mixture comprising hydrophobic substituted cyclic dicarboxylic acid anhydrides and specific polyoxyalkylene alkyl or alkyl-aryl ethers or the corresponding mono- or di-esters. The invention also has as a preferred embodiment a method for sizing paper products directly in the paper stock preparation system without the need for prior emulsification of the size agent.
As used herein, the term "paper and paperboard" includes sheet-like masses and molded products made from fibrous cellulosic materials which may be derived from both natural and synthetic sources. Also included are sheet-like masses and molded products prepared from combinations of cellulosic and non-cellulosic materials derived from synthetics such as polyamide, polyester and polyacrylic resin fibers as well as from mineral fibers such as asbestos and glass.
II. Brief Description of the Prior Art
It is recognized that paper and paperboard are often sized with various hydrophobic materials including, for example, rosin, wax emulsions, mixtures of rosin with waxes, ketene dimers, isocyanate derivatives, fatty acid complexes, fluorocarbons, certain styrene-maleic anhydride copolymers, as well as the substituted cyclic dicarboxylic acid anhydrides more particularly described hereinafter. These materials are referred to as sizes or sizing and they may be introduced during the actual paper making operation wherein the process is known as internal or engine sizing. On the other hand, they may be applied to the surface of the finished web or sheet in which case the process is known as external or surface sizing.
In order to obtain good sizing with any of the previously described sizing compounds, it is desirable that they be uniformly dispersed throughout the fiber slurry in as small a particle size as is possible to obtain. In accordance with the known methods of adding these sizing compounds to the paper stock prior to web formation, therefore, the sizing compound is added in the form of an aqueous emulsion prepared with the aid of emulsifying agents including, for example, cationic or ordinary starches, carboxymethyl cellulose, natural gums, gelatin, cationic polymers or polyvinyl alcohol, all of which act as protective colloids.
These prior art techniques which utilized emulsifying agents with or without added surfactants did, however, suffer from several inherent deficiencies in commercial practice. A primary deficiency concerned the necessity of utilizing relatively complex, expensive and heavy equipment capable of exerting high homogenizing shear and/or pressures, together with rigid procedures regarding emulsifying proportions and temperatures, etc., for producing a satisfactory stable emulsion of the particular size. Additionally, the use of many surfactants in conjunction with protective colloids was found to create operational problems in the paper making process such as severe foaming of the stock and/or loss in sizing.
With particular reference to the procedures of the prior art which utilized substituted cyclic dicarboxylic acid anhydrides as sizing agents, it has been necessary in commercial practice to pre-emulsify with cationic starch or other hydrocolloids using relatively rigid procedures with elevated temperatures to cook the starch or hydrocolloids and high shearing and/or high pressure homogenizing equipment. Unless these complicated procedures are carefully followed, the methods result in process difficulties such as deposition in the paper system, quality control problems and generally unsatisfactory performance.
Although it has been proposed to use such surfactants as polyoxyethylene sorbitan trioleate, polyoxyethylene sorbitol hexaoleate or polyoxyethylene sorbitol oleate-laurate, the use thereof as sole emulsifying agents has not proved satisfactory since it has not been possible to form a stable, small size particle emulsion and/or the emulsion did not produce sufficient sizing performance. Moreover, in some instances, the resultant emulsion detrimentally affected other properties of the sized paper, for example, loss of wet strength. Another drawback noted with various emulsions prepared with certain surfactants, is demonstrated where on aging of the treated paper, many small hydrophilic spots were formed as evidenced by an ink dip test. Thus, the use of these surfactants together with the substituted cyclic dicarboxylic acid anhydrides has been limited to very minor amounts used only in conjunction with protective colloids and under rigorously controlled conditions.
There is thus a need in the art for a method for sizing paper and paperboard products which will eliminate the difficult procedures and complex high shear homogenizing equipment now required for producing an emulsion and will result in more flexibility in preparing and handling the size mixture. There is also a need for a method for producing emulsions of smaller particle size and superior functionality in the paper making process to those prepared by the methods of the prior art, the use of which will result in improved sizing performance and improved operability.